Recently, along development of compact information equipment, electronic components to be mounted have been downsized rapidly. A ball grid alley (hereinafter referred to as “BGA”) having electrodes at its rear surface is applied to such electronic components in order to cope with a narrowed connection terminal and a reduced mounting area because of the downsizing requirement.
As the electronic components to which the BGA is applied, for example, a semiconductor package is exemplified. The semiconductor package is composed with semiconductor chips having electrodes sealed with resin. Solder bump is formed on each of the electrodes of the semiconductor chips. The solder bump is formed by joining a solder ball to an electrode of the semiconductor chip. The semiconductor package to which the BGA is applied is mounted on a printed circuit board by joining the solder bump melted by heating and a conductive land of the printed circuit board. In recent years, a three-dimensional high-density mounting has been developed by stacking up the semiconductor packages in a height direction in order to meet the further high-density mounting requirement.
However, in a case that the BGA is applied to the semiconductor package for the three-dimensional high-density mounting, the solder ball may be crushed by semiconductor package's weight. If such an accident happens, it is conceivable that solder comes out from the electrodes, the electrodes connect to each other, and then a shorted connection occurs.
Accordingly, a solder bump has been considered, where a Cu core ball is electrically joined on an electrode of an electronic component by using solder paste. The Cu core ball comprises a Cu ball which becomes a core of the ball and a solder layer coating the surface of the Cu ball. The solder bump formed by using the Cu core ball can support a semiconductor package by the Cu ball, which is not melted at a melting temperature of the solder, even if the solder bump receives the weight of the semiconductor package when the electronic components are mounted on the printed circuit board. Therefore, it can be prevented that the solder bump is crushed by the semiconductor package's weight.
By the way, in a case where a reflow treatment is conducted on a Cu core ball placed on the electrode of a semiconductor chip, there is an occasion when an oxide film is formed on a solder surface of the Cu core ball due to heating at the time of the reflow treatment. By this influence of the oxide film, wettability defects and the like occur between the solder and an electrode pad. As the result, a mounting failure of the Cu core ball occurs, and therefore a problem that the productivity or yield ratio of the semiconductor package is significantly decreased arises. Accordingly, oxidation resistance is required at the time of melting the Cu core ball and after melting it.
In addition, there is also an occasion when a problem of oxide film of the Cu core ball arises due to the temperature or humidity of the storage environment of the Cu core ball after being manufactured. Even in a case where a reflow treatment is conducted after mounting a Cu core ball having an oxide film formed on the electrodes of the semiconductor package, wettability defects of solder occurs similarly, and the solder constituting the Cu core ball does not wetly spread across the entire electrode. Accordingly, there is a problem that a mounting failure of the Cu core ball occurs because of the electrode exposure, misaligned Cu core ball toward the electrodes or the like. Therefore, the management of oxide film thickness after manufacturing Cu core balls is also an important problem.
For example, Patent Document 1 discloses a technology such that Sn oxide film thickness formed on the surface of a solder ball is controlled at a fixed value or less, by setting yellowness (b* value) of the surface at 10 or less, in the solder ball which is composed of Ag of 0 through 4.0 mass %, Cu of 0 through 1.0 mass %, Sn as a balance and inevitable impurities. Patent Document 2 discloses a technology such that oxidation of Sn is inhibited by forming an oxide film of Ge preferentially on the surface of molten solder, by including Ge of 0.2% or less in a solder alloy.